Combinations of organic inhibitors such as organic phosphonic acids together with polymeric acrylics have been used in aqueous systems. However, the addition of chelating agents to steam generators, for example, to prevent the deposition of metallic ions has been based on the addition of stoichiometric amounts of the chelating agents to the water. These methods require precise control of the chemical addition, since the chelant, for example, if added in less than stoichiometric amounts leads to hardness deposition while any overfeeding has a tendency to cause corrosion due to the by-products generated in the form of insoluble salts which deposit in the system and impair the efficiency of heat transfer.
Other methods attempting to prevent mineral deposits in aqueous systems, i.e. steam generating systems, include the use of phosphonates in combination with various polymeric dispersents. A problem with this combination of materials, however, is that a large volume of suspended solids are formed by the precipitating reaction products which require large blowdowns and therefore reduced efficiency of the system.
In accordance with this invention, the chemical feed problem associated with the fluctuation of the quality of the feed water is designed to prevent an accumulation of excess chelating agent by utilizing a combination of an acrylic acid polymer and polymers derived from maleic acid with at least one organic phosphonate which react with the feed water minerals when the chelant is deficient. It was found that by utilizing a combination of polymers with an organic phosphonate and a chelating agent there was a reduction in the amount of suspended solids with a decrease in the deposition of the various mineral salts resulting in an increase in efficiency which minimizes the needs for maintenance and equipment repair.
There have been various attempts to overcome the problems relating to corrosion and mineral salt deposition in aqueous systems some of which have been solved by the use of various chelating agents in combination with phosphonates as disclosed in U.S. Pat. No. 3,666,664. Here the water-soluble anionic polymers are used in combination with a chelating agent and a phosphonate. Similarly, U.S. Pat. No. 3,959,167 discloses a composition for inhibiting the accumulation of scale on heating surfaces in an aqueous system which comprises an acrylic polymer together with a chelant and an organo phosphonic acid. Further, U.S. Pat. No. 4,255,259 discloses a composition for inhibiting scale in aqueous systems comprising a co-polymer of styrene sulphonic acid with maleic anhydride and a water-soluble phosphonic acid or salt. Similarly, U.S. Pat. No. 3,706,717 discloses that scale deposits can be controlled or prevented in aqueous systems by the addition of acrylic acid polymers in combination with corrosion inhibiting sequestrants. The use of acrylic acid polymers with phosphorous acid products have been suggested for treating aqueous systems by U.S. Pat. Nos. 4,239,648 and 4,159,946.